Systems and subsystems for ensuring proper receipt of a message in a communication system are well-known in the art. Often, such systems are based on two-way communication; a receiver acknowledges receipt of a message, or of one or more submessages, received from a sender or transmitter, the acknowledgment being sent back to the sender or transmitter. Alternatively, a receiver might request re-transmission of messages or submessages which were not received, the absence of such a request being interpreted similarly to an acknowledgment. A particular part of ensuring proper receipt is determining whether or not the message has been properly received.
Systems for sending and receiving scrambled messages are well known in the art. The term “scrambling” in all of its grammatical forms, as used throughout the present specification and claims, refers to any method of scrambling, encoding or encrypting data, many such methods being well-known in the art.
Systems for scrambling television signals, and controlling access thereto, are well known in the art. One such system is described in the following U.S. Pat. No. 5,282,249 to Cohen et al.; and U.S. Pat. No. 5,481,609 to Cohen et al. Scrambled television data streams described in the Cohen et al. patents comprise both scrambled data representing television signals and coded control messages, also known as ECMs. The ECMs of Cohen et al comprise, in a coded form, data necessary for generating a control word (CW) which may be used to descramble the scrambled data representing television signals.
While the two patents to Cohen et al. describe an analog system that is, a system in which analog television data streams are broadcast to television sets, it is appreciated that similar ECM methods may also be used for digital television data streams. Generally, the scrambling techniques used for scrambling analog television signals such as, for example, the well known “cut-and-rotate” technique, are chosen for their applicability to analog signals. In scrambling of digital television signals other scrambling techniques, well-known in the art, are used, the techniques being more appropriate to digital signals such as, for example, applying the well-known DES algorithm to the digital television signals.
It is also known in the art to apply methods analogous to those used for television scrambling to data scrambling particularly in the case of digital methods.
Methods of transmitting a scrambled digital signal, including ECMs, are described in the MPEG-2 standard, ISO/IEC 13818-6, 12 Jul. 1996 and subsequent editions.
Other methods relevant to conditional access techniques employing ECMs are described in the following documents:                ETR 289. “Digital Video Broadcasting (DVB): Support for use of scrambling and Conditional Access (CA) within DVB systems”, and        TS 101 197-1 V1.1.1 (1997-06): “Digital Video Broadcasting (DVB): DVB SimulCrypt; Part 1: Head-end architecture and synchronization”.        
Methods of transmitting ECMs that are particularly suitable for use in a data broadcasting environment are described in applicant's co-pending unpublished Israel patent application 123554, corresponding to PCT international application PCT/IB99/00509, published as WO 99/45711 on 10 Sep. 1999.
It is well-known in the prior art for known forms of ECM processing, including creating and interpreting an ECM, to take place in an IC card, commonly known as a smart card.
The disclosures of all references mentioned above and throughout the present specification are hereby incorporated herein by reference.